Conventional magnetic heads for FDDs, for example, have the constitution described in JP-A-3-256213 (hereinafter referred to as "reference 1"). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") As shown in the FIGS. 1 and 2 given in reference 1, a magnetic head for an FDD generally comprises a head core (electromagnetic converter element) sandwiched between two sliders (reinforcements) bonded thereto.
Ceramics such as calcium titanate and barium titanate are widely used as the materials of such sliders as shown in the FIG. 1 given in reference 1. Use of resins such as poly(phenylene sulfide) resins and polyimide resins as slider materials has been also proposed as shown in the FIG. 2 given in reference 1.
Ceramic sliders are produced through sintering under high-temperature and high-pressure conditions. With respect to resin sliders, use of injection molding has been proposed as a technique for mass-producing sliders of any desired shape, as disclosed in reference 1.
Since resins, when used alone, give sliders inferior in wear resistance and mechanical strength, a filler comprising a powder of an inorganic material such as, e.g., silica or alumina, is used as disclosed in JP-A-59-160860 (hereinafter referred to as reference 2) and JP-A-61-150109 (hereinafter referred to as reference 3).
In reference 2 is disclosed a resin slider containing fine particles of an inorganic material (e.g., alumina, silica, magnesia or zirconia) having a particle diameter of from 0.01 to 5 .mu.m, in an amount of 0.1 to 50 wt %. In reference 3 is disclosed a resin slider containing particles of silica, quartz glass or alumina which have an average particle diameter of from 30 to 150 .mu.m, in an amount of 50 to 90 wt %.
The sliders of conventional magnetic heads for FDDs are made of the materials described above, and have the following drawbacks. Production of sliders made of a ceramic such as calcium titanate or barium titanate necessitates expensive special equipment for high-temperature and high-pressure sintering in order to obtain a dense material.
Another drawback of ceramic sliders is that because of the high hardness of those ceramics, it is difficult to impart intricate shapes to the ceramic materials, resulting in an increased cost of magnetic-head production.
Furthermore, use of ceramic sliders poses a problem that when the magnetic head slides on a floppy disk, the floppy disk sticks to the magnetic head due to the liquefaction of the lubricant contained in the magnetic material of the floppy disk.
On the other hand, sliders made of resins such as poly(phenylene sulfide) resins or polyimide resins have a drawback that since such resins have a remarkably higher coefficient of thermal expansion than Mn--Zn ferrites used as head core materials, bonding of the sliders to a head core material results in a considerable stress imposed on the head core (after bonding in the case where the adhesive used is a thermosetting one, or when the ambient temperature changes in the case where the adhesive used is a cold-setting one), whereby the magnetic characteristics of the head core are significantly deteriorated.
It is therefore essential that a fine powder of an inorganic material having a low coefficient of thermal expansion be used as a filler for a resin used as a slider material.
The coefficients of thermal expansion of resins are generally high as stated above, e.g., from 600.times.10.sup.-7 to 700.times.10.sup.-7 /.degree. C. for thermoset epoxy resins, whereas those of Mn--Zn ferrites are from 100.times.10.sup.-7 to 130.times.10.sup.-7 /.degree. C.
Of the fillers use of which is disclosed in references 2 and 3, magnesia has the highest coefficient of thermal expansion, and the fillers having the next highest coefficient of thermal expansion are zirconia and alumina. These fillers, having coefficients of thermal expansion on the same order as Mn--Zn ferrites (around 100.times.10.sup.-7 /.degree. C.) and having specific gravities several times those of resins, bring about little decrease in the coefficient of thermal expansion when incorporated in amounts up to about 50% by weight and do not contribute to the prevention of the deterioration of head core magnetic characteristics. If those fillers are incorporated in an amount close to 90% by weight as described in reference 3, this impairs the flowability of the uncured resin.
Silica has a far lower coefficient of thermal expansion (from 5.times.10.sup.-7 to 6.times.10.sup.-7 /.degree. C.), and the specific gravity thereof is as low as 2.2. Silica is hence a suitable filler for adjusting the coefficient of thermal expansion to that of a head core. (It should be noted that there are crystalline silica and amorphous silica, and crystalline silica has too high a coefficient of thermal expansion and is hence unsuitable for use in reducing the coefficient of thermal expansion.)
However, silica has a problem that it is inferior to alumina, magnesia, and zirconia in resistance to wearing by floppy disks and in mechanical strength. It is known that the hardness (Vickers hardness) of silica, which is a measure of wear resistance, is as low as about 500, whereas the hardness of alumina is as high as around 2,000 (the hardness of .gamma.-Fe.sub.2 O.sub.3, used as a magnetic material for floppy disks, is about 1,300). However, a magnetic head having a slider containing alumina in an amount exceeding 30% by weight has a problem that it is unsuitable for practical use because the floppy disk sliding on the magnetic head suffers wearing in its sliding part (hereinafter referred to as media wear).